The invention relates in general to reference voltage, and in particular, to a device and method for generating a bias insensitive bandgap voltage.
Reference voltages are utilized in analog circuits extensively. Such reference voltages are precise references that exhibit little or no dependence on process and voltage supply, and have a well defined dependence on the temperature. Various studies and researches have been put forward, in an attempt to realize the zero temperature coefficient, among which a bandgap voltage reference circuit is a popular approach.
For explanatory purposes, two terms are introduced here, namely, positive temperature coefficient and negative temperature coefficient. A positive temperature coefficient quantity denotes a proportional relationship to the absolute temperature, also known as proportional to absolute temperature (PTAT), whereas a negative temperature coefficient quantity represents a counter proportional relationship to the absolute temperature, typically referred to as counter proportional to absolute temperature (CTAT).
The bandgap voltage reference circuit is commonly deployed via the combination of a positive temperature coefficient voltage and a negative temperature coefficient voltage with proper weighing factors, to yield a zero temperature coefficient.
FIG. 1 is a circuit diagram of a conventional bandgap voltage reference circuit 1 comprising the first circuit 10 and the second circuit 12 coupled thereto. The first circuit 10 generates a positive temperature coefficient voltage VPTAT, the second circuit 12 produces a negative temperature coefficient voltage VCTAT. Combining positive temperature coefficient voltage VPTAT and negative temperature coefficient voltage VCTAT renders a bandgap reference voltage Vbg, ideally a constant quantity, irrespective to process, voltage supply, and temperature variation, and is represented by the relationship:Vbg=VPTAT+VCTAT  (1)
The first circuit 10 comprises the first bipolar transistor Q1, the second bipolar transistor Q2, operational amplifier OP1, the first resistor R1, and second resistor R2. The first bipolar transistor Q1 is coupled to the first resistor R1, and then to the non-inverting input of operational amplifier OP1. The second bipolar transistor Q2 is coupled to the inverting input of operational amplifier OP1, such that different emitter-base voltages Veb1 and Veb2 are established across the first bipolar transistor Q1 and the second bipolar transistor Q2 respectively, resulting in an emitter-base voltages difference ΔVeb between Veb1 and Veb2 (ΔVeb=Veb2−Veb1), an inherent positive temperature coefficient voltage, at the output of operational amplifier OP1 coupled to the second resistor R2. Positive temperature coefficient voltage ΔVeb subsequently controls a positive temperature coefficient current IPTAT through the second resistor R2 and establish positive temperature coefficient voltage VPTAT.
The second circuit 12 comprises the third bipolar transistor Q3 coupled to the second resistor R2, and rendering an inherent negative temperature coefficient voltage VCTAT across the emitter and base terminals thereof.
Unfortunately, positive temperature coefficient current IPTAT biasing the bipolar transistor Q3 impairs the performance of negative temperature coefficient voltage VCTAT, since positive temperature coefficient current IPTAT introduces an opposite component in negative temperature coefficient voltage VCTAT and is process and temperature dependent. In practice the positive temperature coefficient current IPTAT may vary up to 20% from integrated circuit (IC) to IC due to process variation. Consequently bandgap reference voltage Vbg can no longer remain at a process and temperature insensitive voltage level. To counter this issue, extra circuit simulation and calibration are incorporated at the expense of production period and circuit complexity, both are undesirable factors in circuit implementation.
Thus, a reference voltage generator and method for generating a bias insensitive reference voltage is proposed.